Protective pads for electrical devices

ABSTRACT

A plurality of elongated lead-in electrodes are hermetically sealed in a wafer of metal oxide varistor material and extend therethrough. The portions of the wafer in contact with said electrodes are spaced to provide a current flow between the electrodes which is low when normal operating voltage appears across the electrodes and when voltages in excess of normal voltage appear progressively thereacross a rapidly decreasing impedance is presented by the wafer in accordance with the alpha of the body of material, thereby limiting the voltage appearing between the electrodes.

United States Patent .1191

Harnden, Jr.

[ 51 July 3,1973

[ PROTECTIVE PADS FOR ELECTRICAL DEVICES [75] Inventor: John D. Harnden,Jr., Schenectady,

[73] Assignee: General Electric Company,

Schenectady, NY.

221 Filed: Oct. 21, 1971 211 Appl. No.: 191,213

[52] US. Cl. 338/21, 317/9 AC, 317/31 [51] Int. Cl H0lc 7/12 [58] Fieldof Search 338/13, 20, 21;

317/238, 31, 9 R, 9 AC, 33 R; 252/461 [56] References Cited UNITEDSTATES PATENTS 3/1937 Lazarus 338/20 3/1962 Rollins et a1 338/20 OTHERPUBLICATIONS Pettus, C. et al., Molybdenum Oxide Negative ResistanceDevices, IBM Technical Disclosure Bulletin Vol. 7, No. 3, August 1964.

Primary Examiner-C. L. Albritton Attorney-John F. Ahern et al.

I S 7 ABSTRACT A plurality of elongated lead-in electrodes arehermetically sealed in a wafer of metal oxide varistor material andextend therethrough. The portions of the wafer in contact with saidelectrodes are spaced to provide a current flow between the electrodeswhich is low when normal operating voltage appears across the electrodesand when voltages in excess of normal voltage appear progressivelythereacross a rapidly decreasing impedance is presented by the wafer inaccordance with the alpha of the body of material, thereby limiting thevoltage appearing between the electrodes.

9 Claims, 9 Drawing Figures 1 PROTECTIVE PADS FOR ELECTRICAL DEVICESPROTECTIVE ELECTRICAL FEED-THROUGH ASSEMBLIES FOR ENCLOSURES FORELECTRICAL DEVICES The present invention relates to electricalfeedthrough assemblies associated with enclosures for providingelectrical connections to electrical devices included in the enclosureand in particular to such assemblies which provide electrical protectionto such devices from surge and other spurious electrical voltages.

Electrical devices such as semiconductor devices are subject toelectrical transient voltages or surges which are developed in thecircuits in which they are used and also which come from externalsources such as lightning, and are conducted to the devices along thepower lines for the devices. Heretofore, a certain degree of protectionagainst such surges has been provided by means of fast acting electricalfuses connected in circuit with the electrical device. Such fuses havenot been entirely satisfactory in view of the fact that a certain timedelay occurs between the occurrence of a surge and the subsequentinterruption of the circuit by the fuse. With fast rising surges oflarge amplitude, such fuses do not act fast enough to protect sensitiveelectrical devices from damage.

Accordingly, an object of the present invention is to provide surgeprotection to electrical devices which is fast acting.

Prior art surge protection techniques required the use of additionalcircuit or structure elements to achieve the desired result.

Another object of the present invention is to provide feed-throughassemblies for enclosures for electrical devices which in addition toproviding a feed-through function also provides the protection againstelectrical surges without the need for separate or additional elements.

Another object of the present invention is to provide a feed-throughassembly which provides good heat dissipation capability as well assurge protection.

In carrying out the present invention, in one illustrative embodimentthereof, there is provided a hermetically sealed electrical feed-throughassembly including a body of metallic oxide varistor material having apair of opposed surfaces. An elongated electrode is provided extendingthrough the body from one of the opposed surfaces to the other of theopposed surfaces and hermetically sealed thereto. Another electrode isprovided in contact with the body and hermetically sealed thereto. Theelectrode may take the form of an elongated electrode, also extendingthrough the body from one of the opposed surfaces to the other of theopposed surfaces. The material has an alpha in excess of in a currentdensity range of IO to 10 amperes per square centimeter. The portions ofthe body in contact with the electrodes are spaced to provide a currentflow between the electrodes which is low when normal operating voltageappears across the electrodes and when voltages progressively in excessof normal voltage appear thereacross, a rapidly decreasing impedance ispresented by the body in accordance with the alpha of the material ofthe body, thereby limiting the voltage appearing between the electrodes.

The novel features which are believed to be characteristic of thepresent invention are set forth in the appended claims. The inventionitself, however, together with further objects and advantages thereofmay best be understood by reference to the following description takenin connection with the accompanying drawings wherein,

FIG. 1 is a perspective view of a feed-through assembly as applied to ahousing or enclosure for a semiconductor device, partially disassembledto show the manner of fabrication thereof.

FIG. 2 is a side view in section of the assembled device of FIG. 1 takenalong section 22 of FIG. 1 showing the construction thereof inaccordance with the present invention.

FIG. 3 shows graphs of the electrical characteristics of three materialsof differing voltage gradients and alphas suitable for utilization inthe feed-through devices of the present invention.

FIG. 4 is a side view in section of another embodiment of the presentinvention.

FIG. 5 is a side view in section of another embodiment of the presentinvention.

FIG. 6 is a side view in section of another embodiment of the presentinvention.

FIG. 7 is a side view in section of a further embodiment of the presentinvention.

FIG. 8 is a bottom view of the device of FIG. 7 taken along sectionlines 8-8 of FIG. 7.

FIG. 9 is a top view of the device of FIG. 7 taken along section lines9-9 of FIG. 7.

Referring now to FIGS. 1 and 2, there is shown a hermetically sealedenclosure 10 for a semiconductor device 11 comprising the headerassembly 12 and a cap member 13. Upon completion of fabrication of theheader assembly 12, flange portion 14 of cap 13 is welded or soldered tomating flange 15 of header assembly 12 to provide a hermetically sealedenclosure for the semiconductor device 11. The header assembly 12, whichprovides the electrical feed-through connections to the semiconductordevice, comprises a base portion 16 in which are embedded a plurality ofelongated conductors or leads 17, 18, and 19 and which is surrounded bya cylindrical conductive member 9 having a flange portion 15 adapted toengage flange portion 14 of the cap 13. The semiconductor device 11 ismounted on a platform or block 20 which may be made of any suitableinsulating material such as ceramic or glass and is provided with aplurality of holes 21, 22, and 23 extending therethrough from one majorsurface or face to the opposite major surface thereof and in addition isprovided with a centrally located slot 24. On major face of the platformis also provided with a plurality of metallized areas 25, 26, and 27,two of which are contiguous to the slot 24 and each of which adjoins arespective hole. The semiconductor device 11, shown in this casein theform ofa bar of semiconductor material which has a pair of regions ofone conductivity type and an intermediate region of the oppositeconductivity to form a transistor device, is conductively secured at itsends to the metal oxide areas contiguous to the slot by means of asuitable solder. A flexible wire-like conductor 29 is fused to theintermediate region or portion of the bar 11 and soldered to themetallized area 27. The ceramic block 20 is then securely held in placeon the insulating base portion 16 by a solder bond between each of theleads 17, 18, and 19 and the metallized portions 25, 26, and 27. Thepresence of the slot 24 in the ceramic platform 20 ensures separation ofthe conductive areas of the ceramic platform and hence also assures thatthe intermediate region of the bar 11 will be free of conductive contactwith either of the metallized areas 25 and 26. While a mounting assemblyfor a particular semiconductor device has been shown for inclusion in ahermetically sealed enclosure with a portion of the enclosure beingprovided with feed-through leads, such assembly is included simply forpurposes of illustration of the manner of application of the presentinvention.

The base portion 16 of the header assembly is in the form of a waferhaving a pair of opposed major faces with the leads 17, 18, and 19extending through the wafer from one major face to the opposite majorface thereof and hermetically sealed thereto.

The base portion or wafer 16 is constituted of a metal oxide varistormaterial such as described in Canadian Pat. No. 831,691, which has anonlinearvoltage versus current characteristic. The metal oxide varistormaterial described in the aforementioned patent is constituted of fineparticles of zinc oxide with certain additives which have been pressedand sintered at high temperatures to provide a composite body or waferof material. The current versus voltage characteristics of the compositebody is expressed by the following equation:

I (V/C a 1 where V is voltage applied across a pair of opposed surfacesor planes,

I is the current which flows between the surfaces,

C is a constant which is a function of the physical dimensions of thebody as well as its composition and the process used in making it,

a is a constant for a given range of current and is a measure of thenonlinearity of the current versus voltage characteristic of the body.

In equation (1), when V is used to denote voltage between opposedsurfaces or planes of a unit volume of material, or voltage gradient,current flow through the unit volume of material in response to thevoltage gradient becomes current density. For the metal oxide varistormaterial for current densities which are very low, for example, in thevicinity of a mircroampere per square centimeter, the alpha (a) isrelatively low, i.e., less than 10. In the current density range of fromto 10 amperes per square centimeter, the alpha is high, i.e.,substantially greater than 10 and relatively constant. In the currentdensity ranges progressively in excess of 10 amperes per squarecentimeter, the alpha progressively decreases. When the current versusvoltagecharacteristic is plotted on log-log coordinates, the alpha isrepresented by the reciprocal of the slope of the graph in which currentdensity is represented by the abscissa and voltage gradient isrepresented by the ordinate of the graph. For a central range of currentdensities of from 10' to 10 amperes per square centimeter, thereciprocal of the slope is relatively constant. For current densitiesbelow this range, the reciprocal of the slope of the graph progressivelydecreases. Also for current densities above this range, the reciprocalof the slope of the graph progressively decreases.

The voltage gradient versus current density characteristics of threetypes of material in log-log coordinates are set forth in FIG. 3. Graphs30 and 31 are materials of high voltage gradient material and graph 32is a graph of low voltage gradient material. For all of the graphs inthe current density range from 10 to 10 amperes per square centimeter,the alpha is high and is substantially greater than 10 and relativelyconstant. For current densities progressively greater than l0 per squarecentimeter, the alpha progressively decreases. For current densitiesprogressively less than 10" per square centimeter, the alpha alsoprogressively decreases.

As the metal oxide varistor material is a ceramic material, the surfacesthereof may be metallized for facilitating electrical connectionsthereto in a manner similar to the manner in which other ceramicmaterials are metallized. For example, Silver Glass Frit, DuPont No.7713, made by the DuPont Chemical Company of Wilmington, Delaware, maybe used. Such material is applied as a slurry in a silk screeningoperation and fired at about 550 C to provide a conductive coating onthe surface. Other methods such as electroplating or metal sprayingcould be used as well.

The nonlinear characteristics of the metal oxide varistor materialresults from bulk phenomenon and is bidirectional. The response of thematerial to steep voltage wave fronts is very rapid. Accordingly, thevoltage limiting effect of the material is practically instantaneous.Heat generation occurs throughout the body of material and does notoccur in specific regions thereof as in semiconductor junction devices,for example. Accordingly, the material has good heat absorptioncapability as the conversion of electrical to thermal energy occursthroughout the body of material. The specific heat of the material is0.12 caloric/C/gram. Accordingly, on this account, as well, heatabsorption capability of the material is advantageous as a surgeabsorption material. The heat conductivity of the material is aboutone-half of the heat conductivity of alumina. Accordingly, any heatgenerated in the material may be rapidly conducted from the materialinto appropriate heat sinks.

The material, in addition to the desired electrical and thermalcharacteristics described above, has highly desirable mechanicalproperties. The material has a fine grain structure, may be readilymachined to a smooth surface and formed into any desired shape havingexcellent compressive strength. The material is readily molded in theprocess of making the same. Accordingly, any size or shape of materialmay be readily formed for the purposes desired. Also, the coefficient ofexpansion of metal oxide varistor material is comparable to thecoefficient of expansion of glass, silica, and the like.

In the case of the base portion 16 of metal oxide varistor material,holes are machined into the material of appropriate size to receive theelectrodes or leads. The leads 17, 18, and 19 may be of electrode stocksuch as are used conventionally for electrodes or leads for use in glassor ceramic headers having a coefficient of expansion comparable to thecoefficient of expansion of glass. The interior regions of the holes aremetallized as indicated by cylinders 7 and 8 by one of the processesdescribed above. The leads are inserted in place in the face member andthe material fired to provide a hermetic seal.

The materials which may be used for the header enclosure are materialswhich have temperature coefficient of expansion matching that of glass.Such material may be alloys of iron, nickel, and cobalt of variousproportions well known in the art for this purpose. One such material isfernico having a coefficient of expansion similar to the coefficient ofexpansion of glass and which hermetically bonds thereto.

The leads 17, 18, and 19 in the metal oxide varistor material basemember 16 are spaced from one another and from the header enclosure 9 toprovide high impedance at normal operating voltages between any two suchelectrodes for the specific metal oxide varistor material utilized. Ofcourse, when the spacings of the leads with respect to one another arefixed by standard practice, the particular metal oxide varistor materialutilized is selected to provide the desired voltage versus currentcharacteristic which limits the amplitude of transient voltage surgeswhich may appear across any of a pair of leads or a lead and theexternal casing 9.

Referring now to FIG. 4, there is shown another embodiment of thepresent invention similar to the embodiment of FIGS. 1 and 2 andidentical elements are identically designated. The base member 34 of theheader assembly 12 is made of a suitable glass and is bonded to ametallic supporting plate 35 which has a plurality of large openings 36,37, and 38 therein, each adapted to register with respective electrodeor leads 17, 18, and 19 of the device. The plate 35 may be made of amaterial such as fernico which has a thermal coefficient of expansionmatching that of glass and ceramic. A plurality of small cylinders 40,41, and 42 of metal oxide varistor material are provided for leads 17,18, and 19, respectively. The inner diameter of a cylinder is madeslightly greater than the outer diameter of a lead and the outerdiameter of a cylinder is made slightly smaller than the inner diameterof the apertures of the plate 35. The cylinders 40, 41, and 42 areconductively secured along their inner cylindrical surfaces torespective leads 17, 18, and 19 and their outer cylindrical surfaces areconductively secured to the surfaces of the respective apertures 36, 37,and 38 in plate 35. The material and dimensions of each of theindividual cylindrical elements is selected to provide the propervoltage versus current characteristic between the electrical leads 17,18, and 19 of the device such that the current flow between the leads islow when normal operating voltages appear across the electrodes and whenvoltages in excess of normal voltage progressively appear thereacross arapidly decreasing impedance is presented by the cylinders in accordancewith the alpha of the body of material, thereby limiting the voltageappearing between the leads and the casing of assembly 12.

Reference is now made to FIG. 5 which shows a modification of the deviceof FIGS. 1 and 2 in which each of the elements of FIG. 5 identical tothe elements of FIG. 2 are designated by the same reference numeral. Inthis figure, the outer exposed surface of the base portion 16 metaloxide varistor material is covered with a glass layer 45 which may beapplied by any of a variety of techniques known in the art to providefurther sealing against environmental influences. In feed-throughassemblies in which the conductive contact between the electrodes of theassembly, which may include conductive casings, and the body of metaloxide varistor material do not provide a hermetic seal, insulatingcoatings of a plastic such as epoxy as well as glass may be used toprovide the hermetic seals.

Reference is now made to FIG. 6 which shows another embodiment of thepresent invention, which is similar to the embodiment of FIG. 2, withthe additional feature that a conductive glass sealing material 50provides the conductive and hermetic bond between the body of metaloxide varistor material and each of leads 17, 18, and 19.

Reference is now made to FIG. 7 which shows another embodiment inaccordance with the present invention, which is similar to theembodiment of FIG. 5, with the additional provision of a plurality ofconductive layers or strips secured to the opposed surfaces of the metaloxide varistor body or wafer to enable greater flexibility to beachieved with respect to the voltage versus current characteristicsbetween any pair of conducting electrodes. The elements of FIG. 7,identical to the elements of FIG. 4, are designated by the samenumerals. In this figure, the metal oxide varistor wafer 16 has appliedto one surface a pair of conductive strips 60 and 61, first strip 60extending from the lead 18 and terminating in a straight edge 62 and asecond strip 61 extending from the lead 19 also terminating in astraight edge 64 to form a gap with the first strip. The separation ofthe adjacent straight edges 62 and 64 of the strips 60 and.61 are set toprovide the desired voltage versus current characteristic between thestrips which limits the amplitude of transient voltage surges which mayappear across the leads l8 and 19. The opposite surface of the wafer isalso provided with metal strips. Metal strip 65 is in conductive contactwith the electrode 18, extends a distance therefrom and terminates in astraight edge 66. Similarly, a conductive strip 67 extends from the lead17 along a straight line between lead 17 and lead 18 and terminates in astraight edge 68 intermediate the distance between the two leads to forma gap with straight edge 66. Conductive strip 70 extends from the lead17, extends a distance toward lead 19 and terminates in a straight edge71. Similarly, strip 72 extends from the lead 19 and terminates in astraight edge 73 to form a gap therewith. The spacing of strips 65 and67 is arranged to provide the desired voltage versus currentcharacteristic between the leads l8 and 17 which limits the amplitude oftransient voltage surges which may appear across leads 18 and 17.Similarly, the gap between strips 70 and 72 is arranged to provide adesired voltage versus current characteristic between the leads l7 and19. Metal oxide varistor structures utilizing laterally spacedelectrodes are also described and claimed in my copending patentapplication, Ser. No. 165,001, Metal Oxide Varistor with LaterallySpaced Electrodes, filed July 22, 1971 and assigned to the assignee ofthe presnt application.

While the invention has been described in specific embodiments, it willbe appreciated that modifications may be made by those skilled in theart and I intend by the appended claims to cover all such modificationsas fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A hermetically sealed electrical feed-through assembly comprising abody of metallic oxide varistor material having a pair of opposedsurfaces,

an elongated electrode extending through said body from one opposedsurface to the other opposed surface thereof and hermetically sealedthereto by metallic means surrounding said electrode and in contact withsaid body,

another electrode in conductive contact with said body.

2. The combination of claim 1 in which said material has an alpha inexcess of 10 in the current density range of 10 to l amperes-per squarecentimeter.

3. The combination of claim 1 in which the portions of said body incontact with said electrodes are spaced to provide a current flowbetween said electrodes which is low when normal operating voltageappears across said electrodes and when voltages in excess of normalvoltage progressively appear thereacross a rapidly decreasing impedanceis presented by said body in accordance with the alpha of the body ofmaterial thereby limiting the voltage appearing between said electrodes.

4. The combination of claim 1 in which said other electrode is anelongated electrode and extends through said body from one opposedsurface to the other opposed surface thereof.

5. The combination of claim 1 in which said other electrode is aconductive member which surrounds said body.

6. The combination of claim 1 in which one of said surfaces is providedwith an insulating coating.

7. An electrical feed-through assembly comprising a wafer of metallicoxide varistor material having a pair of opposed surfaces,

a plurality of elongated electrodes, each extending through said waferfrom one opposed surface to the other opposed surface thereof and inconductive contact therewith,

said material having an essentially constant alpha in excess of l0 inthe current density range of 10' to 10 amperes per square centimeter,

the portions of said wafer in contact with said electrodes being spacedto provide a current flow between a pair of said electrodes which is lowwhen normal operating voltage appears across said pair of electrodes andwhen voltages in excess of normal voltage appear thereacross a rapidlydereasing impedance is presented by said wafer in accordance with thealpha of the material of said wafer thereby limiting the voltageappearing between said pair of electrodes.

8. The combination of claim 7 in which one of said surfaces is providedwith a pair of spaced conductive layers, each in conductive contact witha respective one of a pair of said electrodes, the distance between saidlayers along said one surface being set to obtain a desired normaloperating point on the voltage versus current graph of one pair ofelectrodes.

9. The combination of claim 8 in which the other of said surfaces isprovided with another pair of spaced conductive layers, each inconductive contact with a respective one of another pair of saidelectrodes, the distance between said other pair of layers along saidother surface being set to obtain a desired normal operating point onthe voltage versus current graph of said other pair of electrodes.

2. The combination of claim 1 in which said material has an alpha inexcess of 10 in the current density range of 10 3 to 102 amperes persquare centimeter.
 3. The combination of claim 1 in which the portionsof said body in contact with said electrodes are spaced to provide acurrent flow between said electrodes which is low when normal operatingvoltage appears across said electrodes and when voltages in excess ofnormal voltage progressively appear thereacross a rapidly decreasingimpedance is presented by said body in accordance with the alpha of thebody of material thereby limiting the voltage appearing between saidelectrodes.
 4. The combination of claim 1 in which said other electrodeis an elongated electrode and extends through said body from one opposedsurface to the other opposed surface thereof.
 5. The combination ofclaim 1 in which said other electrode is a conductive member whichsurrounds said body.
 6. The combination of claim 1 in which one of saidsurfaces is provided with an insulating coating.
 7. An electricalfeed-through assembly comprising a wafer of metallic oxide varistormaterial having a pair of opposed surfaces, a plurality of elongatedelectrodes, each extending through said wafer from one opposed surfaceto the other opposed surface thereof and in conductive contacttherewith, said material having an essentially constant alpha in excessof 10 in the current density range of 10 3 to 102 amperes per squarecentimeter, decreasing the portions of said wafer in contact with saidelectrodes being spaced to provide a current flow between a pair of saidelectrodes which is low when normal operating voltage appears acrosssaid pair of electrodes and when voltages in excess of normal voltageappear thereacross a rapidly decreasing impedance is presented by saidwafer in accordance with the alpha of The material of said wafer therebylimiting the voltage appearing between said pair of electrodes.
 8. Thecombination of claim 7 in which one of said surfaces is provided with apair of spaced conductive layers, each in conductive contact with arespective one of a pair of said electrodes, the distance between saidlayers along said one surface being set to obtain a desired normaloperating point on the voltage versus current graph of one pair ofelectrodes.
 9. The combination of claim 8 in which the other of saidsurfaces is provided with another pair of spaced conductive layers, eachin conductive contact with a respective one of another pair of saidelectrodes, the distance between said other pair of layers along saidother surface being set to obtain a desired normal operating point onthe voltage versus current graph of said other pair of electrodes.